[CMake] Further comment *why* there are Xcode-specific warning options.
[CMake] Further comment *why* there are Xcode-specific warning options.

Also remove option for -Wmissing-braces, since it's not clear yet
the warning is desired.

Swift SVN r15029

airspeedswift/swift,practicalswift/swift,arvedviehweger/swift,karwa/swift,jtbandes/swift,manavgabhawala/swift,manavgabhawala/swift,karwa/swift,tinysun212/swift-windows,rudkx/swift,devincoughlin/swift,arvedviehweger/swift,russbishop/swift,huonw/swift,alblue/swift,allevato/swift,swiftix/swift.old,dduan/swift,emilstahl/swift,KrishMunot/swift,djwbrown/swift,milseman/swift,adrfer/swift,ahoppen/swift,return/swift,JaSpa/swift,lorentey/swift,apple/swift,swiftix/swift,jtbandes/swift,kusl/swift,zisko/swift,gmilos/swift,deyton/swift,hughbe/swift,kperryua/swift,parkera/swift,jckarter/swift,OscarSwanros/swift,huonw/swift,Jnosh/swift,frootloops/swift,tkremenek/swift,shajrawi/swift,uasys/swift,hughbe/swift,roambotics/swift,tjw/swift,harlanhaskins/swift,ken0nek/swift,gribozavr/swift,mightydeveloper/swift,mightydeveloper/swift,gmilos/swift,aschwaighofer/swift,lorentey/swift,uasys/swift,harlanhaskins/swift,kusl/swift,sschiau/swift,jmgc/swift,bitjammer/swift,JaSpa/swift,adrfer/swift,shajrawi/swift,xedin/swift,khizkhiz/swift,roambotics/swift,natecook1000/swift,cbrentharris/swift,shajrawi/swift,austinzheng/swift,hooman/swift,OscarSwanros/swift,sdulal/swift,OscarSwanros/swift,russbishop/swift,sdulal/swift,aschwaighofer/swift,kstaring/swift,tjw/swift,adrfer/swift,harlanhaskins/swift,natecook1000/swift,rudkx/swift,manavgabhawala/swift,LeoShimonaka/swift,uasys/swift,airspeedswift/swift,roambotics/swift,swiftix/swift,LeoShimonaka/swift,tjw/swift,practicalswift/swift,tjw/swift,kperryua/swift,sschiau/swift,cbrentharris/swift,airspeedswift/swift,Jnosh/swift,shahmishal/swift,johnno1962d/swift,khizkhiz/swift,codestergit/swift,uasys/swift,sdulal/swift,jtbandes/swift,codestergit/swift,tkremenek/swift,natecook1000/swift,swiftix/swift.old,jopamer/swift,KrishMunot/swift,felix91gr/swift,brentdax/swift,ken0nek/swift,manavgabhawala/swift,manavgabhawala/swift,milseman/swift,devincoughlin/swift,johnno1962d/swift,russbishop/swift,OscarSwanros/swift,CodaFi/swift,cbrentharris/swift,tinysun212/swift-windows,ahoppen/swift,shahmishal/swift,zisko/swift,felix91gr/swift,devincoughlin/swift,hooman/swift,gmilos/swift,glessard/swift,alblue/swift,khizkhiz/swift,codestergit/swift,khizkhiz/swift,gregomni/swift,swiftix/swift,atrick/swift,atrick/swift,roambotics/swift,JaSpa/swift,karwa/swift,amraboelela/swift,MukeshKumarS/Swift,ben-ng/swift,gribozavr/swift,natecook1000/swift,mightydeveloper/swift,jmgc/swift,amraboelela/swift,xwu/swift,jtbandes/swift,jckarter/swift,johnno1962d/swift,aschwaighofer/swift,apple/swift,return/swift,shajrawi/swift,frootloops/swift,LeoShimonaka/swift,practicalswift/swift,gottesmm/swift,arvedviehweger/swift,karwa/swift,IngmarStein/swift,Ivacker/swift,therealbnut/swift,adrfer/swift,brentdax/swift,hooman/swift,JaSpa/swift,adrfer/swift,gregomni/swift,deyton/swift,huonw/swift,sdulal/swift,parkera/swift,LeoShimonaka/swift,glessard/swift,johnno1962d/swift,parkera/swift,bitjammer/swift,tjw/swift,benlangmuir/swift,huonw/swift,alblue/swift,uasys/swift,Ivacker/swift,devincoughlin/swift,bitjammer/swift,alblue/swift,therealbnut/swift,jckarter/swift,Ivacker/swift,hughbe/swift,devincoughlin/swift,sschiau/swift,JGiola/swift,sdulal/swift,Jnosh/swift,LeoShimonaka/swift,codestergit/swift,amraboelela/swift,SwiftAndroid/swift,jmgc/swift,uasys/swift,brentdax/swift,stephentyrone/swift,harlanhaskins/swift,practicalswift/swift,kentya6/swift,djwbrown/swift,huonw/swift,modocache/swift,stephentyrone/swift,gribozavr/swift,lorentey/swift,MukeshKumarS/Swift,shajrawi/swift,kperryua/swift,parkera/swift,slavapestov/swift,ben-ng/swift,xedin/swift,deyton/swift,ben-ng/swift,stephentyrone/swift,arvedviehweger/swift,emilstahl/swift,slavapestov/swift,tardieu/swift,harlanhaskins/swift,alblue/swift,danielmartin/swift,russbishop/swift,nathawes/swift,practicalswift/swift,modocache/swift,jckarter/swift,JaSpa/swift,return/swift,lorentey/swift,frootloops/swift,benlangmuir/swift,dreamsxin/swift,stephentyrone/swift,ahoppen/swift,nathawes/swift,nathawes/swift,jtbandes/swift,gribozavr/swift,arvedviehweger/swift,JGiola/swift,danielmartin/swift,IngmarStein/swift,JGiola/swift,jmgc/swift,djwbrown/swift,gottesmm/swift,ken0nek/swift,kstaring/swift,frootloops/swift,jopamer/swift,xedin/swift,dduan/swift,frootloops/swift,hughbe/swift,stephentyrone/swift,danielmartin/swift,gottesmm/swift,benlangmuir/swift,harlanhaskins/swift,jckarter/swift,karwa/swift,karwa/swift,devincoughlin/swift,jopamer/swift,ahoppen/swift,kstaring/swift,roambotics/swift,slavapestov/swift,calebd/swift,milseman/swift,xwu/swift,deyton/swift,frootloops/swift,kusl/swift,xedin/swift,mightydeveloper/swift,djwbrown/swift,glessard/swift,airspeedswift/swift,swiftix/swift,JaSpa/swift,sschiau/swift,kstaring/swift,calebd/swift,CodaFi/swift,ahoppen/swift,OscarSwanros/swift,emilstahl/swift,ben-ng/swift,sschiau/swift,CodaFi/swift,swiftix/swift.old,therealbnut/swift,airspeedswift/swift,natecook1000/swift,emilstahl/swift,MukeshKumarS/Swift,tkremenek/swift,LeoShimonaka/swift,Ivacker/swift,kusl/swift,atrick/swift,aschwaighofer/swift,danielmartin/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